Until recently, the cathode ray tube ("CRT") has been the primary device for displaying information. While having sufficient display characteristics with respect to color, brightness, contrast and resolution, CRTs are relatively bulky and power hungry. These failings, in view of the advent of portable laptop computers, has intensified demand for a display technology which is lightweight, compact, and power efficient.
One available technology is the flat panel display, and more particularly, the liquid crystal display ("LCD"). LCDs are currently used for laptop computers. However, LCDs provide poor contrast in comparison to CRT technology. Further, LCDs offer only a limited angular display range. Moreover, color LCD devices consume power at rates incompatible with extended battery operation. In addition, a color LCD type screen tends to be far more costly than an equivalent CRT.
In light of these shortcomings, there have been several developments recently in thin film, field emission display ("FED") technology. In U.S. Pat. No. 5,210,472, commonly assigned with the present invention, and incorporated herein by reference, a FED design is disclosed which utilizes a matrix-addressable array of pointed, thin-film, cold cathode emitters in combination with a conductive, transparent screen having a conductive coating which is in turn, coated with a cathodoluminescent material. An extraction grid having a plurality of openings aligned with respective emitters is positioned between the emitters and the screen. The screen is biased at a relatively high voltage on the order of 80 V to 1 KV. When the voltage of the extraction grid is sufficiently higher than the voltage of the emitters, electrons are emitted from the underlying emitter and are attracted to the conductive screen. When the electrons strike the cathodoluminescent material, light is emitted at the point of impact. The intensity of the emitted light is proportional to the rate at which electrons are emitted which is, in turn, proportional to the voltage differential between the extraction grid and emitter. The FED incorporates a column signal to activate a single column extraction grid, while a row signal activates a row of emitters. At the intersection of both an activated column and an activated row, a grid-to-emitter voltage differential exists sufficient to induce electron emission. Extensive research has recently made the manufacture of an inexpensive, low power, high resolution, high contrast, full color FED a more feasible alternative to LCDs.
In order to achieve the advantages of this technology, as in the performance of LCDs, FED devices require a brightness control scheme. Several techniques have been proposed to control the brightness and gray scale range. For example, U.S. Pat. No. 5,103,144 to Dunham and U.S. Pat. No. 5,103,145 to Doran, both incorporated herein by reference, teach methods for controlling the brightness and luminance of flat panel displays. However, a need remains for a brightness control scheme that requires less power and is simpler to manufacture. Further, a need exists for a brightness control scheme requiring less circuitry and thus less surface area on a silicon die.